Changes in the shape of joint bones are a possible cause of arthritic joint diseases. Bone anomalies can occur in the region of the pelvic joint and can lead to bone interferences in the joint region. When the leg moves over time these interferences can cause bone surfaces to wear. Computer-assisted planning can be used to prepare reconstruction treatments that restore a suitable bone shape. Computer-assisted planning has been based on determining the contour of joint bones on a two-dimensional basis (for example, in a particular incision plane). The reference “Journal of Bone and Joint Surgery”, Volume 84-B, No. 4, May 2002, pages 556 to 560, Nötzli et al. discloses determining a contour of a femoral neck, in which a measurement system and/or angle system is specified. Nötzli et al. is incorporated herein by reference in its entirety.